In the current business environment within North America, the majority of drilling rigs have shifted away from vertical to horizontal well drilling in order to more effectively exploit low quality shale reservoirs. Such reservoirs have extreme vertical heterogeneity, with individual vertical layers ranging in thickness from tenths of an inch to a few inches, while horizontally the reservoir tends to remain quite consistent. In this environment, placement of the wellbore within the reservoir can be the difference between a successful well and an economic failure. Therefore, successful stimulation of these reservoirs is paramount to achieve economic production rates.
However, successful stimulation first requires optimal placement of the wellbore along the vertical strata. Locating the wellbore in a soft, ductile and, thus, unproductive region can make stimulation and long term production very difficult due to the high fracture initiation pressures and loss of fracture conductivity under production conditions. Also, stress transition regions can have a significant impact on fracture growth, thus making it difficult to contact an adequate amount of the reservoir. Thus, the goal is to locate the wellbore in a portion of the shale reservoir that will maximize the effectiveness of a stimulation treatment. Such a desired portion would be in close proximity to the stored hydrocarbon, contain brittle rock that can be easily stimulated with fracturing, comprise a simple stress regime that will allow adequate fracture growth, and have a high natural fracture density.
For a number of reasons, conventional geosteering approaches have had limited success in locating the wellbore within the desired high-producing portions of the reservoir. First, the drilling assembly is essentially driven blind, as engineers and geologists at the surface rely on down hole sensors and telemetry to provide data about the rock quality around the bit. Once the data is received at the surface, the drilling team must then interpret the data, and adjust the assembly accordingly—a very time consuming process. Second, the amount of downhole data that can be transmitted back to the surface using current telemetry methods is severely limited given the roughly 100 KB maximum data transmission rate of current telemetry methods. This, in addition to the shear distance from the surface to the drill bit, often results in a drastic lag time in the geosteering response.
Third, given the slow data transmission rate, the drilling team often is forced to wait until the drill bit actually contacts a surface before taking corrective action. As a result of these lag inducing factors, the wellbores are often tortuous and fail to remain in the optimal section of the reservoir. Fourth, in very layered reservoirs, current geosteering approaches simply lack the precision required to achieve the accuracy necessary for optimal wellbore placement. Lastly, conventional geosteering assemblies are not forward-looking; instead, they simply react to the received downhole data.
Accordingly, in view of these disadvantages, there is a need in the art for a highly-responsive, forward-looking and precise geosteering assembly, thus resulting in optimal placement and quality of the wellbore.